Early electronics used vacuum tubes to amplify, switch, and modulate electrical signals. Many decades ago, most vacuum tubes were replaced by solid-state devices such as diodes and metal-oxide-semiconductor field-effect transistors (MOSFETs). The transition from vacuum tubes to solid-state devices was not driven by the superiority of the semiconductor as a carrier transport medium, but by the ease of fabrication, low cost, low power consumption, light weight, long lifetime, and ideal form factor of solid-state integrated circuits (ICs). While vacuum tubes were fabricated by mechanical machining and used as discrete components, modern solid-state devices are batch processed into integrated circuits. Additionally, cathodes of conventional vacuum tubes need to be heated for thermionic emission of electrons, and the energy for heating adversely overwhelms the energy required for field emission. A conventional vacuum device is, therefore, not suitable for low power applications.
However, a conventional solid-state semiconductor transistor does not perform well in extreme environments, such as very high temperature or where radiation is present. A vacuum device is more robust than a solid-state device in extreme environments involving high temperature and exposure to radiation. And for high power amplification (e.g., >50 W), a solid-state device requires a complex circuit architecture including many transistors, microstrips, and thermal management systems. A device that operates in a vacuum or near-vacuum offers immunity to radiation, increased robustness, and relatively high frequency, power input, and power amplification, but consumes more energy for similar performance. The critical tradeoff is that vacuum tubes yield higher frequency/power output but consume more energy than MOSFETs.
Transport in a vacuum is intrinsically superior to transport in a solid medium, because vacuum transport allows ballistic transport while solid-state carriers suffer from optical and acoustic phonon scattering in semiconductors. More specifically, charged particle carrier transport in a conventional solid-state transistor is dominated by a drift-diffusion mechanism, with an associated transport velocity limit of about 5×107 cm/sec (depending on the type of semiconductor used). Electrons are scattered and high temperature operation often results in a reduction of drive current. In contrast, electrons in a vacuum can move with few or no collisions (depending on the level of vacuum). A vacuum channel transistor, which relies upon thermionic emission and quantum tunneling, can operate ballistically for carrier transport, with a theoretical ballistic transport velocity of about 3×1010 cm/sec.